The invention relates to circuitry and techniques for reducing distortion in the output stage of push-pull amplifiers, preventing bandwidth reduction when a high speed output transistor is turned off in favor of conduction in a slower output transistor, and making the output impedance of the output stage more steady than in prior push-pull amplifier output stages.
Push-pull amplifier stages characteristicly produce distortion due to transistors that turn off as part of the push-pull amplifier operation. Furthermore, the turning off of a high speed output transistor of a push-pull amplifier in favor of increasing current conduction in a slower output transistor of the push-pull amplifier typically results in a reduction in bandwidth and a change in the output impedance of the push-pull amplifier.
The output turn off mechanism of a push-pull amplifier stage can be described with reference to the typical push-pull amplifier circuit 1 of FIG. 1. In this so-called complementary circuit, an NPN pullup transistor 2 and a resistor 4 are connected as an emitter follower to supply an output current i.sub.o through conductor 5, and PNP pulldown transistor 7 is connected with resistor 6 to sink an output current (-i.sub.o) through conductor 5. In order to ease the transition from current "sourcing" of output current by pullup transistor 2 and current "sinking" of output current by pulldown transistor 7 a quiescent bias voltage V.sub.B is applied between the bases of transistors 2 and 7. Usually, bias voltage V.sub.B is developed across diodes such as 9 and 10, which themselves are biased by current source 3.
Bias voltage V.sub.B is absorbed in varying degrees by the current "sourcing" pullup transistor 2 and the current "sinking" pulldown transistor 7 depending on the instantaneous output current i.sub.o. When i.sub.o is zero, the current flowing in transistor 2 and resistor 4 also flows in transistor 7 and resistor 6. Then the sourcing and sinking sides of circuit 1 each "absorb" approximately one-half of V.sub.B.
When an appropriate value of an input drive signal e.sub.i makes i.sub.o positive, more of bias voltage V.sub.B is required to "support" pullup transistor 2 in its "on" condition and less of V.sub.B is available to bias pulldown transistor 7 in its "on" condition. Therefore, an increase in the current through transistor 2 results in a drop in the current flowing through transistor 7. In the circuit of FIG. 1, the only way that current supplied by transistor 2 can be increased is to decrease the portion of the bias voltage V.sub.B absorbed by transistor 7. This operation completely turns transistor 7 off in response to relatively moderate increases in the current through transistor 2. Similarly, an opposite operation turns off transistor 2 when i.sub.o is negative and transistor 7 is sinking output current. The current through transistor 2 is indicated by curve 2A in FIG. 2, and the current through transistor 7 is indicated by curve 7A in FIG. 2.
The above described push-pull operation produces signal distortion because of nonlinearity of transistors 2 and 7 and mismatches between their operating parameters. The nonlinearity results from the exponential current-voltage relationship of the transistor emitter-base junctions of output transistors 2 and 7. As one of these transistors is turning on and the other is turning off, the distortion due to the emitter-base response reaches its maximum, resulting in crossover distortion. Distortion also results from mismatches in the characteristics of transistors 2 and 7. For example, differences in current gains of transistors 2 and 7 cause a change in the output resistance as the output current i.sub.o reverses polarity.
To avoid such distortion, output stages have been developed that ensure continuous bias of the output transistors. Such bias is shown for the complementary emitter follower stage disclosed in U.S. Pat. No. 3,995,228, in which the bias voltage applied between the bases of the emitter follower transistors is increased as output current increases. This is achieved by means of diodes that couple the circuit output voltage to the bias circuit. Similar circuits are shown in U.S. Pat. No. 4,274,059 (Okabe) and U.S. Pat. No. 4,401,951 (Tanaka). However, the complementary emitter follower stage of such circuits requires both NPN and PNP output transistors; this is frequently undesirable because most integrated circuit manufacturing processes cannot readily produce PNP transistors suitable for the higher current levels needed in the output stage. For this reason, many push-pull amplifier output stages are formed of two output transistors of the same conductivity type. Such stages are push-pull in nature, with fixed bias sources. Therefore, transistor turnoff and its distortion are present in such noncomplementary push-pull output circuit stages.
Such a stage is adapted for continuous bias in U.S. Pat. No. 4,573,021 (Widlar). There, the current in one of the output transistors is sensed and compared to a reference current. The difference found by that comparison is used to drive a feedback loop that adjusts the bias to keep the sensed transistor continuously on. However, that technique greatly increases the complexity of the output stage. For example, implementation of this technique requires addition of a differential amplifier, a reference current, a reference diode, and a sensed diode. Furthermore, this circuit forms a feedback loop containing multiple gain stages each of which adds phase delay and reduces bandwidth.
In "noncomplementary" prior art push-pull amplifier circuits, the collector of the pulldown transistor normally would be connected directly to the output terminal. Under those conditions, the output current has no effect on the bias current of the pullup transistor. The need to turn one of transistors 2 and 7 off in order to turn the other one on strongly is illustrated by the two output transistor collector current curves 2A and 7A in FIG. 2. One of the output devices 2 or 7 always is turned off when there is a significant excursion of the output current i.sub.o, obviously resulting in the crossover distortion referred to above. It should be appreciated that the "classic" complementary output stage shown in FIG. 1 is difficult to implement in integrated circuits because it is difficult to manufacture integrated circuit PNP transistors that have good characteristics under high current conditions. Integrated circuit push-pull amplifiers generally are therefore noncomplementary circuits.
Thus, the most relevant known prior art deals with the output transistor turn off problem by altering the fixed bias source, either a constant bias voltage for complementary implementations or a fixed bias current source for noncomplementary implementations. These techniques use feedback from the output to the fixed bias source in order to increase the potential of the bias source under conditions of high current conduction in one of the output transistors.